Photoinduced Proton Transfer Promoted by Peripheral Subunits for

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Photo-Induced Proton Transfer Promoted by Peripheral Subunits for Some Hantzsch Esters Sébastien Azizi, Gilles Ulrich, Maud Guglielmino, Stéphane Le Calvé, Jerry Hagon, Anthony Harriman, and Raymond Ziessel J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp5078246 • Publication Date (Web): 04 Dec 2014 Downloaded from http://pubs.acs.org on December 10, 2014

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The Journal of Physical Chemistry

Photo-induced Proton Transfer Promoted by Peripheral Subunits for Some Hantzsch Esters Sébastien Azizi,a Gilles Ulrich,a Maud Guglielmino,b Stéphane le Calvé,b Jerry P. Hagon,c Anthony Harriman,*c and Raymond Ziessel*a (a) Institut de Chimie et Procédés pour l’Energie, l’Environnement et la Santé (ICPEES), Laboratoire de Chimie Moléculaire et Spectroscopies Avancées (ICPEES-LCOSA), UMR 7515 au CNRS, Ecole Européenne de Chimie, Polymères et Matériaux, 25 rue Becquerel, 67087 Strasbourg Cedex 02, France. (b) ICPEES - UMR 7515 CNRS - UdS. Institut de Chimie, Procédés, Catalyse, Energie, Environnement et Santé, Physico-chimie de l'atmosphère, 1 rue Blessig, 67084 Strasbourg Cedex 02, France. (c) Molecular Photonics Laboratory, School of Chemistry, Bedson Building, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK

AUTHOR EMAIL ADDRESS: [email protected] RECEIVED DATE TITLE RUNNING HEAD: Long-range Proton Transfer CORRESPONDING AUTHOR FOOTNOTE: [email protected]: [email protected]

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ABSTRACT: It is noted that, for a small series of 3,5-diacetyl-1,4-dihydrolutidine (DDL) derivatives and the corresponding Hantzsch esters, the presence of methyl groups at the 2,6-positions serves to extinguish fluorescence in solution but not in the solid state. Emission is weakly activated and affected by changes in solvent polarity. The latter situation arises because the optical transition involves intramolecular charge transfer. Calculations, both semi-empirical and DFT, indicate that, in all cases, rotation of the carbonyl function is facile and that the dihydropyridine ring is planar. These calculations also indicate that the 2,6-methyl groups do not affect the generic structure of the molecule. It is proposed that illumination increases the molecular dipole moment and pushes electron density towards the carbonyl oxygen atom. Proton transfer can now occur from one of the methyl groups, leading to formation of a relatively low-energy, neutral intermediate, followed by a second proton transfer step that forms the enol. Reaction profiles computed for the ground-state species indicate that this route is highly favored relative to hydrogen transfer from the 4-position. The barriers for light-induced proton transfer are greatly reduced relative to the ground-state process but such large-scale structural transformations are hindered in the solid state. A rigid analogue that cannot form an enol is highly emissive in solution, supporting the conclusion that proton transfer is in competition to fluorescence in solution.

KEYWORDS: Hantzsch esters / fluorescence / transition state / hydrogen transfer / keto-enol

INTRODUCTION Formaldehyde is one of the more harmful air pollutants1,2 due to its relatively high atmospheric concentration in urban environments and its deleterious impact on human health. Indeed, it has been classified as carcinogenic by IARC.3 Such concerns mean that there is a need for reliable but straightforward analytical protocols to enable the routine monitoring of formaldehyde vapors; in practical terms, such analyses are most conveniently performed after extraction of gaseous formaldehyde into water.1,4-7 In seeking to develop sensitive ways to monitor the presence of formaldehyde in aqueous solution, we were drawn to the in-situ formation of fluorescent 3,5-diacetyl-1,4-dihydrolutidine (DDL) 2 Environment ACS Paragon Plus

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derivatives.8-16 Although this earlier work focused mostly on identification of water-soluble DDL derivatives, it was noted that the fluorescence properties of these compounds were highly sensitive to the nature of substituents in close proximity to the ring nitrogen atom. We set out now to examine the origin of this observation.

O

O

O

O

EtO DD L

OEt R

N H

N H

R

H an t zs c h e s t e r s R = H , M e , p h e n y l , t o ly l

Chart I. Generic formulae for the DDL-type derivatives and the corresponding Hantzsch esters described herein. In so doing we note that these DDL derivatives, being members of the 1,4-dihydropyridine family, are closely related to the so-called Hantzsch esters, which are already known to display solvent-dependent fluorescence spectral properties.17 Both DDL and Hantzsch esters (Chart I) can be formed by reactions of formaldehyde under rather mild conditions and might be adapted as fluorescent markers for the presence of this analyte in solution.18 To aid the mechanistic studies, a small series of 1,4dihydropyridine derivatives has been prepared and fully characterized. Included within this series is a rigid derivative that inhibits internal rotation of the carbonyl functions. It is evident that the Hantzsch esters exhibit the same relationship between fluorescence quantum yield and molecular topology as observed for the DDL derivatives. Specifically, the presence of methyl groups at the 2,6-positions extinguishes emission in solution but not in the solid state. There is no obvious structural reason for this observation.

RESULTS and DISCUSSION The target compounds are depicted in Chart II and make use of complementary synthetic approaches to prepare the DDL derivatives and the corresponding Hantzsch esters. In particular, a new synthetic 3 Environment ACS Paragon Plus


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procedure was devised, starting from propargylic esters, for the synthesis of compounds 1 and 2. A more conventional procedure was adapted to prepare the 1,4-dihydropyridine derivatives 3-6. The former method makes use of a condensation reaction involving the propargylic ester with para-formaldehyde and ammonium acetate, as is illustrated by way of Scheme 1. It might be noted that this latter reaction, which is performed under mild conditions, has particular relevance for the introduction of an analytical protocol for detection of formaldehyde. The intention is to develop this routine as a means by which to establish viable fluorescence-based tests for the presence of formaldehyde.

Chart II. Chemical formulae and reference numbers for the compounds examined in this contribution. It should be noted that the pivotal propargylic ester was prepared according to Scheme 2. Here, the tolyl ester A requires deprotonation of the acetylenic unit, followed by reaction with ethyl chloroformate. Preparation of the 1,4-dihydropyridine derivatives follows a standard procedure based on condensation of the appropriate acetylacetonate derivative with para-formaldehyde and ammonium


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acetate, as is outlined by way of Scheme 3. Reaction yields are high and isolation of the final product is straightforward.

Scheme 1. General synthetic protocol used for preparation of compounds 1 and 2. O OEt

i) nBuLi, -78°C ii) ethylchloroformate, -78°C

A

Scheme 2. Outline of the methodology employed for the preparation of precursor A.

Scheme 3. Outline of the Hantzsch synthesis adapted to prepare the target compounds, starting from either the acetylacetone derivative or the dimedone substrate.



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In the absence of X-ray crystal structural data, quantum chemical calculations (DFT/B3LYP/6311G**/CPCM) were performed in order to establish lowest-energy conformations for these compounds. The first important feature relates to the conclusion19,20 that the six-member ring, in each compound, is in essence planar (Figure 1). This observation, which is in excellent agreement with an earlier X-ray crystallographic study,21,22 is consistent with the inclusion of dipolar resonance forms as part of the overall electronic signature of the molecular structure. It might be noted that the presence of aryl groups at the 4-position causes the ring to buckle.23,24 Molecular planarity of our compounds is supported by the observation that the C4-N-H angle is very nearly 1800 in each case. Other informative structural outputs include the N-C2, C-O and C2-C3 bond lengths as compiled in Table 1, and the corresponding dihedral angles between the ring and the carbonyl residues.

Figure 1. Energy-minimized structure computed (DFT/B3LYP/6-311G**/CPCM) for 4 (N blue, O red, C gray and H white). We can begin a discussion of these findings by reference to the DDL-like analogue 4. Here, the lowest-energy conformation has the two carbonyl groups directed towards the lower rim of the planar dihydrolutidine ring (Figure 1); again, this observation is in excellent agreement with a prior crystal structure determination.21,22 The dihedral angles between the ring and carbonyl groups are close to 00. However, internal rotation, calculated25 by DFT (B3LYP/cc-pVTZ0) methods, is facile, and the structure with one carbonyl group pointing away from the ring is destabilized by only ca. 1 kJ/mol (Figure 2). This “instability” is caused by slight steric crowding due to the presence of hydrogen atoms 6 Environment ACS Paragon Plus

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on the proximal methyl groups. The computed structure having both carbonyl groups pointing away from the ring is only marginally (i.e.,

Photoinduced Proton Transfer Promoted by Peripheral Subunits for

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